It's possible you ran into problems with large pathtags, fixed by [235], but that hasn't been merged yet. With that and some other tweaks (we still have to manually adjust buffer sizes, but that will soon be dynamic again as well), we are able to render the paris-30k test file, which has ~50k paths and almost a million path segments.
We will also soon be special-casing rectangles. Might be worth rerunning your experiment in a few weeks when all this has landed.
In any case, glad for your interest!